1. Field of the Invention
This invention relates to an improved method of producing aluminum alloy sheet products by twin roll casting and processing the sheet to a final automotive body sheet. More specifically, it relates to such a method which produces sheet having improved formability and strength.
2. Description of the Prior Art
With the increased emphasis on the desirability of reducing the weight of motorized vehicles, such as automobiles, vans, trucks and sport utility vehicles, there has been increased motivation to replace steel body panels of vehicles with lighter aluminum panels. In order to make such a transformation appealing, the strength and formability of the aluminum sheet alloys must be comparable to that of steel sheet and must be economically competitive. Typically in the prior art practices, aluminum would be cast into an ingot of approximately 12 to 28 inches in thickness, which ingot would be scalped and preheated, after which it might be hot rolled to about 0.125 inch, cold rolled to about 0.020 to 0.060 inch, and subjected to further heat treatment, such as batch annealing or solution heat treatment. One of the problems with such an approach has been the size of the intermetallic particles present in the as-cast aluminum ingots which is a characteristic of the alloy composition and solidification rate in the casting process. These particles can participate in the fracture initiation and propagation process and, as a result, limit formability or design tolerance properties. The intermetallic particles act as void nucleation sites during sheet forming processes, such as stretching and, therefore, contribute to fracture initiation.
The twin roll casting process produces aluminum sheet containing smaller intermetallic particles than the ingot route, as a result of the higher solidification rate. The process produces heavy sheet thicknesses which may be on the order of about 0.200 to 0.350 inch. See Nes et al., xe2x80x9cCasting and Annealing Structures in Strip Cast Aluminum Alloy,xe2x80x9d Aluminium 55, (5), pp. 319-324 (1979). Aluminum sheet cast by this method is cast at a relatively slow speed which may be on the order of 50 to 80 inches per minute, thereby resulting in relatively low productivity. This low productivity offsets most of the cost savings that would otherwise be achieved by employing twin roll casting processes. Conventional post-caster processing usually includes one or more batch-type anneal or homogenization steps in which the cooling rate following these thermal treatments is extremely slow and may be on the order of about 50xc2x0 F. per hour. This slow cooling rate permits significant precipitation of phases containing soluble alloy elements, such as magnesium and copper, which if left in the solid solution in the aluminum matrix, would increase the strength and might contribute to enhanced formability of the finished product.
U.S. Pat. No. 4,441,933 discloses the production of aluminum sheets suitable for drawing wherein the roll cast product is subjected to mechanical brushing or subjected to a jet of gas in a cleaning treatment, after which it is subjected to batch annealing or continuous annealing.
U.S. Pat. No. 4,186,034 discloses casting an aluminum magnesium alloy having a zinc addition in order to render the alloy deformable in the soft condition and suitable for motor vehicle body components. The process includes hot rolling the ingot followed by cold rolling, annealing, further cold rolling, and cooling. See, also, U.S. Pat. No. 4,753,685.
U.S. Pat. No. 4,808,247 discloses an aluminum-silicon-magnesium-copper alloy for use in automotive body components to provide the desired strength and formability.
In spite of the foregoing disclosures, there remains a substantial need for an effective process of producing aluminum alloy sheet for automotive applications in a more economical manner while having the desired formability and strength characteristics.
The present invention has met the hereinabove described needs. In a preferred practice of the present invention, an aluminum alloy strip is created by roll casting and has a thickness of less than about 0.5 inch and, preferably, less than about 0.20 and, most preferably, about 0.040 inch to 0.20 inch. The roll cast strip could then be cold rolled to the final sheet thickness for the specific application which will generally be less than about 0.15 inch and preferably about 0.020 to 0.15 inch and, must preferably, about 0.020 to 0.040 inch. The cold rolled sheet then is subjected to continuous annealing, preferably at a temperature above 750xc2x0 F., but below the melting temperature of the alloy for about 1 to 60 seconds. A preferred use of the panels produced in accordance with this embodiment of the invention is in the formation of internal body structural panels for motor vehicles.
In another embodiment of the invention, in lieu of the continuous annealing, one may subject the cold rolled sheet to solution heat treatment at a temperature above the solvus temperature, but below the melting temperature of the alloy at a temperature of about 800xc2x0 F. to 1100xc2x0 F. for about 1 to 60 seconds and, preferably, about 2 to 30 seconds. Such panels exhibit high strength and are particularly suited for use in body panels.
The method preferably does not employ thermal treatment between casting and cold rolling.
Among the alloys suitable for use in the continuous annealing embodiment of the invention are alloys of the AA 3XXX and 5XXX series. Among the alloys suitable for use in the solution heat treatment embodiment of the invention are the AA 2XXX and 6XXX series.
It is an object of the present invention to provide a process for producing aluminum alloy sheet having equal or improved combinations of strength and formability at lower manufacturing cost.
It is another object of the present invention to produce such strip which has desired surface quality and may be created while eliminating certain prior art thermal treatment steps in order to minimize the cost of the sheet manufacturing process.
It is another object of the present invention to provide a method of producing aluminum motor vehicle body sheet.
It is a further object of the present invention to produce aluminum alloy sheet which is uniquely suited in respect of strength, formability and surface properties for use in motor vehicle body panels.
These and other objects of the invention will be more fully understood from the following detailed description of the invention.
As employed herein, the term xe2x80x9cmotor vehiclexe2x80x9d shall include automobiles, vans, trucks, and sport utility vehicles.
While for simplicity of disclosure herein, specific reference will be made to motor vehicle body panels, it will be appreciated that the process of the present invention may be employed advantageously to create sheet suitable for numerous other uses. For example, for motor vehicle outer body panels, dent resistance is an important characteristic which depends on yield strength of the panel. Also, it is desirable to have the body panel sheet harden during the thermal treatment designed to cure the paint. For body panels, high formability is also a desired property. For motor vehicle structural sheet, the sheet should have desired stiffness, formability and weldability. TABLE I illustrates currently preferred ranges, respectively, for the outer body panels and body structural sheet.
It will be appreciated from the data which follows that the sheet produced by the present invention satisfies these standards.
In a preferred practice of the invention, for producing an aluminum alloy sheet having high strength and formability with desired surface quality, an aluminum strip is twin roll cast to a thickness of less than about 0.5 inch, preferably less than about 0.20 inch and, most preferably about 0.040 to 0.20 inch, after which, if desired, it is subjected to cold rolling to produce a sheet having a final thickness of about 0.020 to 0.15 inch and, preferably, about 0.020 to 0.040 inch. Subsequently, the cold rolled sheet is subjected to continuous annealing at a temperature of between 750xc2x0 F. and the melting temperature of the alloy for about 2 to 30 seconds. In a preferred practice of the invention, the roll cast strip will have a thickness of about 0.040 to 0.20 inch, and the strip will be reduced by cold rolling to a sheet having a thickness of about 0.020 to 0.040 inch. In the preferred practice of the invention, the cold rolling is effected without intermediate thermal treatment, hot rolling, or surface cleaning between the roll casting and cold rolling. There is no required hot rolling step employed prior to the cold rolling. Some hot worked structure may be present in the strip as a result of the roll casting process. The continuous annealing is effected in order to reduce the yield strength and render the material more formable. The continuous annealing may be effected at about 750xc2x0 F. to the aluminum alloy melting temperature for about 1 to 60 seconds and, preferably, about 800xc2x0 F. to 950xc2x0 F. for 2 to 15 seconds. The sheet is then preferably cooled to protect the surface.
This approach of the present invention is to be contrasted with the conventional approach of casting an ingot of about 12 to 28 inches in thickness followed by scalping, preheating, homogenizing, hot rolling to approximately 0.125 inch, after which cold rolling and annealing may be employed.
Among the alloys which may be employed in the continuous anneal process of the present invention are alloys of the 3XXX series and 5XXX series. Examples of specific suitable alloys are 5042, 5052 and 5182, for example.
After employing this process, conventional means may be employed to convert the aluminum alloy sheet into the desired motor vehicle panel.
In a related embodiment of the invention, after casting in the manner hereinbefore described, cold rolling may be effected to about 0.020 to 0.15 inch and, preferably, about 0.020 to 0.080 inch, followed by solution heat treatment rather than continuous annealing. The solution heat treatment may be effected at a temperature of about 800xc2x0 F. to 1100xc2x0 F. for about 1 to 60 seconds and, preferably, about 950xc2x0 F. to 1050xc2x0 F. for about 2 to 15 seconds in order to cause one or more of the alloying constituents to enter solid solution. This is followed by sufficiently rapid cooling as to resist undesired constituent precipitation. Such cooling may be effected, for example, by forced air, water spray or water mist.
Among the alloys suitable for use in the heat treatment embodiment of the invention are alloys of the 2XXX series and 6XXX series. Examples of suitable alloys for use in the solution heat treatment embodiment of the invention are 2008, 2036, 6009 and 6111.